Electrolysis of Ti2CO solid solution prepared by TiC and TiO2

Article abstract of DOI:10.1016/j.jallcom.2006.08.016

TiO₂ can be reduced by TiC at temperatures in excess of 1000 °C, under vacuum conditions. The resulting product was found to show the structures of a Ti₂CO solid solution, which has excellent conductivity like a metal. A series of experiments have been performed on the possibility of titanium electrolysis, using a Ti₂CO solid solution as an anode, in a NaCl-KCl melt. Carbon monoxide (CO) was monitored at the anode during electrolysis when the potential was kept constant. The product on the cathode was analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that titanium powders can be prepared through electrolysis of a Ti₂CO solid solution. The oxygen content of the titanium powders was measured and the result shows that it is lower than 300 ppm.

Full text of DOI:10.1016/j.jallcom.2006.08.016

Journal of Alloys and Compounds 438 (2007) 243–246
Electrolysis of Ti2CO solid solution prepared by TiC and TiO2
Shuqiang Jiao, Hongmin Zhu∗
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China
Received 1 June 2006; received in revised form 7 August 2006; accepted 9 August 2006
Available online 22 September 2006
Abstract
TiO2 can be reduced by TiC at temperatures in excess of 1000 ◦C, under vacuum conditions. The resulting product was found to show the
structures of a Ti2CO solid solution, which has excellent conductivity like a metal. A series of experiments have been performed on the possibility
of titanium electrolysis, using a Ti2CO solid solution as an anode, in a NaCl–KCl melt. Carbon monoxide (CO) was monitored at the anode during
electrolysis when the potential was kept constant. The product on the cathode was analyzed using scanning electron microscopy (SEM) and X-ray
diffraction (XRD). The results show that titanium powders can be prepared through electrolysis of a Ti2CO solid solution. The oxygen content of
the titanium powders was measured and the result shows that it is lower than 300 ppm.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Titanium; Ti2CO solid solution; Electrolysis; Consuming anode
1. Introduction
gated due to their potential for decreasing the cost of titanium
production.
Titanium and titanium alloys have attracted much attention
because of their many applications, including in corrosion pro-
tection, electrode for metal production and for producing shape
memory alloy, etc. The primary titanium metal, spongy tita-
nium, is currently produced industrially by a process invented
by Kroll [1]. The Kroll process is a batch method including two
steps, involving the conversion of titanium oxide to titanium
tetrachloride and titanium tetrachloride reduction by pure mag-
nesium metal. The specific cost of titanium metal produced by
the Kroll process is maintained high due to the expensive steps
required to obtain the metal.
Since the early 1970s, the research focus of the titanium
industry has been directed towards developing a continuous pro-
cess to produce high purity titanium, with a low production cost
[2–8]. Manykindsofalternativemethodshavebeeninvestigated.
Electrolysis, the extraction and preparation of pure metal from
ores using an electrolytic process, is considered as a promising
method for titanium metallurgy [4–8]. Recently, several electrol-
ysis and relative methods such as the FFC Cambridge process
[9–13] and calciothermic reduction [14–17] have been investi-
It is well known that both titanium carbide (TiC) and titanium
monoxide (TiO) have the same cubic structure and space group-
ing, and a similar lattice size, so they are capable of forming a
continuous series of solid solutions. Wainer et al. obtained the
mutual solid solution of titanium carbide and titanium monox-
ide through dissolving titanium carbide and titanium monoxide
using an arc temperature in excess of 2000 ◦C [18,19]. The solid
solution can be extracted to form the titanium ion in the molten
salt, using the electrolysis conditions. The carbon and oxygen
component in the solid solution will liberate to form the CO gas.
This work presents a continuous electrolysis process for pro-
ducing pure titanium. A Ti2CO solid solution prepared by TiO2
and TiC at a temperature in excess of 1000 ◦C, and this was used
asananodein NaCl–KClmelt. Carbonmonoxidewasmonitored
on the electrode during the electrolysis when the potential was
kept constant. Furthermore, pure titanium metal was obtained
on the cathode.
2. Experimental details
TiO2 and TiC powders of reagent grade were mixed with the mole ratio of
1:2 and pressed at 300–1000 kg cm−2 into a bar. The bar was sintered at vari-
ous temperature from 600 ◦C to 1200 ◦C, under vacuum (<100 Pa) for 4 hours.
The chloride salt mixture NaCl–KCl was melted in an alumina crucible placed
within a hermetic alumina tube inside a furnace controlled by a CHINO DZ3000
∗
Corresponding author. Tel.: +86 10 62332267; fax: +86 10 62334204.
E-mail address: hzhu@metall.ustb.edu.cn (H. Zhu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.08.016

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S. Jiao, H. Zhu / Journal of Alloys and Compounds 438 (2007) 243–246
programmable device. The molten salt mixture was dehydrated under flowing
argon at 300 ◦C and then fused under an argon atmosphere at 800 ◦C. The melt
temperature was measured using a thermocouple protected by an alumina tube.
Electrochemical experiments were performed with a potentiostat/galva-
nostat (EG&G Princeton Applied Research, Model 263A). The reference elec-
trode was a Ag/AgCl electrode. This consists of a silver wire with a 1 mm
diameter immersed in a NaCl–CsCl molten mixture containing 4 wt.% silver
chloride, contained in a mulite tube. The potential of the reference electrode was
calibrated against Cl2/Cl− electrode generated by currentstatic electrolysis on
a graphite bar. A potential range of −1.0 V to −0.2 V versus Cl2/CI− reference
was used to perform potentiostatic electrolysis. A gas chromatograp (Hewlett
packard 5880 A series) was applied to monitor the anode gas. The structure and
morphology of the cathode product was analyzed by X-ray diffraction (XRD)
and scanning electron microscopy (SEM).
3. Results and discussion
Fig. 2. A typical current to time profile performed using an electrolytic poten-
tiostatic value of −0.45 V vs. Cl2/Cl−.
TiO2 and TiC powders were mixed homogenously and
pressed to form pellets with diameter of 20 mm and height of
10 mm. The pellet herein before was sintered at various tem-
peratures. XRD analysis was used to study the structure of the
sintered pellets, and the corresponding result is shown in Fig. 1.
In the pellets sintered at 600 ◦C, there still remains strong peaks
of raw materials (TiO2 and TiC), while in the pellets sintered at
1000 ◦C, peaks for TiO2 almost disappear. Those results indicate
that the reduction reaction of TiO2 by TiC happens at temper-
atures higher than 1000 ◦C. The reduction reaction of TiO2 to
TiO by TiC could be described as,
Table 1
The anode gas concentration at different potential conditions
Potential (V) (vs. Cl2/Cl−)
Anode gas component (ppm)
CO
CO2
−l.0 (rest)
−0.65
0
600
0
0
0
0
−0.45
4300
7600
−0.25
2TiO2 + TiC → 3TiO + CO(g)
(1)
result from the formation of compound or solid solution struc-
ture of TiO with TiC (TiC·TiO) , which reduces the activity of
TiO sharply and promotes the reaction. Therefore, the product
of the reduction is not pure TiO but the solid solution. In fact
when we start from the mole ratio of 1:2 for TiO2 and TiC, the
product formation is Ti2CO. To further present this variation, the
conductivity of the pellets was measured, the results show that
the conductivity of the sintered pellets sharply increase (>10,000
times), which might be because of the changing crystal structure
from titanium dioxide to a Ti2CO solid solution.
The reaction equilibrium constant of the reaction, K1 = Pco,
could be calculated from the standard free energy of the com-
pounds involved, and the results of K1, at 600, 1000 and 1200 ◦C,
are 1.4 × 10−11, 2.9 × 10−5 and 2.7 × 10−3, respectively. Obvi-
ously, it should be impossible to reduce TiO2 by TiC to pure TiO
through equation (1), at 1000 ◦C and even at 1200 ◦C, under the
vacuum level of this research (10−3 atm.). However, our experi-
mental results clearly demonstrated the progress of the reduction
at the temperatures higher than 1000 ◦C. This phenomenon may
The pellets of the Ti2CO solid solutions were used as the
anode, carbonsteelasthecathodeandaAg/AgClelectrodeasthe
Fig. 3. Typical square wave voltammograms for background (solid line) and
titanium ion dissolved from the Ti2CO solid solution (dot and solid line) in
NaCl–KCl melt.
Fig. 1. XRD traces of the prepared products at various temperatures using TiC
and TiO2 as reactants.

S. Jiao, H. Zhu / Journal of Alloys and Compounds 438 (2007) 243–246
245
reference electrode. Potentiostatic electrolysis was performed
in NaCl–KCl molten salt at 800 ◦C. Gas-spectra apparatus was
used to measure the off-gas. The relationship between time and
current is presented in Fig. 2.
Table 1 shows the concentrations of the anode gas at the
same sampling time when various electrolysing potentials were
applied. It was found that CO gas was produced during elec-
trolysis, and the concentration of it increased with increasing
potential until chlorine gas evolution started. The phenomenon
may result from the electrochemical extraction of carbon and
oxygen at the anode potentials.
Titanium carbide has been used as a consumable anode to
obtain highly pure titanium. However, carbon residue from the
TiC anode disturbed the continuous electrolysis. In contrast, in
the case of the solid solution anode, the carbon component of
the solid solution anode can be extracted as carbon monoxide
gas with the oxygen component. The titanium component of the
anode was thought to dissolve into the molten salt as a titanium
ion. In order to confirm this expectation, the experiment was
carried out by using a used melt. A tungsten microdisk elec-
trode with a diameter 0.1 mm was used as working electrode, a
graphite bar with a diameter of 6 mm as the counter electrode
and the same Ag/AgCl reference electrode as detailed above.
Fig. 3 shows the result obtained by a negative potential sweep
through square wave voltemmetry. One wave appears at a poten-
tial of −1.6 V versus Cl−/Cl2. This wave is responsible for
the reduction of the titanium ion dissolved into the melt dur-
ing potentiostat electrolysis. A well Gaussian shape will be
beneficial to calculate the exchanging electron number of the
electrochemical reduction from the width of the wave, accord-
ing to the theory of Osteryoung et al [20]. The wave width was
obtained by curve fitting with Guanssian peak, and resulting
the electron number of 2.07. That means that the titanium ion
dissolved by the anode electrolysis is Ti2+.During the electroly-
sis, titanium deposit happens on the cathode. The pure titanium
was obtained on the cathode by constant current electrolysis on
a steel bar with a diameter of 6 mm for several hours. Fig. 4a
shows the morphology of the titanium powder. A grain size of
more than 40 ␮m was found. Fig. 4b presents the XRD trace of
powder electrodeposited on the cathode, and it can be seen that
the powder shows the crystal structure of pure titanium. Oxy-
gen content of the cathode product was measured and the result
showed that it was lower than 300 ppm.
4. Conclusion
Ti2CO solid solution was prepared through the reaction of
TiO2 and TiC at temperatures in excess of 1000 ◦C, under vac-
uum conditions. The solid solution obtained shows excellent
conductivity. A series of experiments have been conducted on
the possibility of titanium electrolysis using a Ti2CO solid solu-
tion as an anode. Carbon monoxide CO evolved at the anode
was monitored during the electrolysis when the potential was
kept constant. The product on the cathode was investigated by
scanning electrode microscopy with EDS, and X-ray diffraction.
The results show that titanium powders can be prepared through
electrolysis of a Ti2CO solid solution. Oxygen content of the
titanium powders was measured and it was lower than 300 ppm.
Acknowledgements
Support for this work was provided by the National Natural
Science Foundation of China through contract NO. 50374007
and 50574012.
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